CAMBRIDGE, Mass. — It's an intense and highly unusual morning here in the Boston metro area. Residents of Watertown, Newton, Belmont, Allston-Brighton, Cambridge, and all of Boston have been ordered by city and state officials to "shelter in place" while a massive manhunt for a second suspect in the Boston Marathon bombings, identified as 19-year-old Dzhokhar Tsarnaev, continues.

The metro area is being described as being "on lockdown." But what does being on lockdown actually mean in a major American city, where widely applied shelter-in-place protocols are virtually unheard of (and as CBS News points out, normally reserved for chemical or biological attacks)? The MBTA, the city's mass transit agency, has halted all service, and Boston taxi drivers have been ordered off the roads. Many local businesses and nearly all local schools and universities are closed. Still, a walk through the middle of Cambridge this morning shows just how difficult it is to actually enforce a citywide lockdown even in the most dangerous of circumstances.

Around 9 a.m. this morning, normally busy Harvard Square was indeed strangely quiet. But close to two dozen people, some of them homeless, some confused tourists wondering why the entrance to the T was closed, plus a handful of taxi drivers with nowhere to go, lingered out in the open with no apparent objections from two uniformed Cambridge PD officers who stood watch on the corner. A newsstand and a CVS remained open in the square as law enforcement vehicles with sirens blaring sped past every couple of minutes. A handful of pedestrians and civilian cars made their way through nearby sidestreets without being stopped.

With every available officer assigned to the manhunt (and there are far more law enforcement officials here in town than normal given the ongoing investigation into Monday's bombing events), in Cambridge at least, there's no sense of being under any sort of "martial law" scenario. Residents are free to move about, they're just being urged not to do so.